Ellipsodial mirror

ABSTRACT

An optical system capable of providing a magnification which can be varied comprises a pair of ellipsoidal mirrors ( 20, 21 ) which are so mounted and arranged that the focus for radiation reflected from one mirror corresponds substantially with the input focus for the other mirror. The mirrors are preferably mounted so that they are pivotable.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application is a divisional application of currentlypending U.S. patent application No. 09/942,019, filed Aug. 29, 2001.

FIELD OF THE INVENTION

[0002] This invention relates to optical systems capable of providing amagnification which can be varied.

BACKGROUND OF THE INVENTION

[0003] Optical systems which can provide a variable magnification areoften required and have been known for a long time. In the visibleregion of the electromagnetic spectrum, variable magnification opticalsystems have been implemented for many years using a series of lenseswhose separations can be varied in order to change the magnificationprovided by the lens system. A problem with lens systems is that becauseof their refractive nature chromatic aberrations can occur and this typeof lens system often requires a number of extra elements in order toovercome this problem. At wavelengths outside the visible region, forexample in the infrared region, the number of materials which have thenecessary transmission qualities to form such lenses is severelyrestricted and this restricted choice of material can have the effect ofreducing the performance of the optical system that can be achieved overa range of wavelengths. Also the cost of the system is increased.

[0004] This problem has been dealt with in the past by constructingvariable magnification optical systems entirely of mirrors. An advantageof a mirror system is that there is no chromatic aberration. However adisadvantage of many mirror systems is that each mirror generally worksoff axis and as a consequence aberrations occur and increase rapidly asthe light propagates through the system of mirrors. Another disadvantageof many mirror systems is the complexity of the mechanical system thatis required in order to move the individual mirrors in order to achievea change in magnification. This arises because the mirrors are notpositioned and moved on a common axis as in the case of lenses and theposition of the mirrors has to be controlled in two rather than onedimension.

SUMMARY OF THE INVENTION

[0005] The present invention is concerned with an optical system whichhas been developed in order to deal with both the aberration andmechanical articulation problems associated with the prior art.

[0006] According to the present invention there is provided an opticalsystem capable of providing a magnification which can be variedcomprising a pair of ellipsoidal mirrors which are so mounted andcoupled that the point at which radiation reflected from the firstmirror is focused corresponds substantially with the input focus of thesecond mirror. The mirrors may be mounted for pivotable movement so thatthe angle between their axes can be varied.

[0007] The optical system may include a first pair of ellipsoidalmirrors and a second pair of ellipsoidal mirrors, said pairs of mirrorsbeing so mounted and coupled that the output focus of the first paircorresponds substantially to the input focus of the second pair. Themirrors may be coupled by articulation means and the articulationbetween the output mirror of the first pair and the input mirror of thesecond pair is at the output focus of the first pair. The said point ofarticulation may be constrained to move along a predetermined path whenthe mirrors are moved in order to achieve a change in magnification.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The invention will be described now by way of example only, withparticular reference to the accompanying drawings. In the drawings:

[0009]FIGS. 1 and 1a illustrate the operation of ellipsoid mirrors;.

[0010]FIG. 2 illustrates schematically a first embodiment in accordancewith the present invention;

[0011]FIG. 2a illustrates the arrangement of FIG. 2a in a differentconfiguration;

[0012]FIG. 3 illustrates another embodiment of the system in accordancewith the present invention, and

[0013]FIG. 3a shows the system of FIG. 3 in a different configuration.

DETAILED DESCRIPTION

[0014] Embodiments in accordance with the present invention provideoptical systems which have continuously variable magnification and arebased upon the use of ellipsoidal mirrors. An ellipsoidal mirror has theproperty that any ray that passes through one of its foci will bereflected to emerge through its other focus. This is illustrated in FIG.1 and la of the drawings. An object placed at the focus (10) of theellipsoidal mirror (11) will generate an image at the focus (12). Thedegree of magnification provided by the mirror varies in accordance withthe angle at which the rays impinge upon the surface of the surface ofthe mirror. FIG. 1 shows an arrangement which can achieve highmagnification, whilst the arrangement of Figure la achieves a lowermagnification. When using a single mirror such as the arrangements shownin FIGS. 1 and 1a, aberrations, which appear in the image, increaserapidly as the point at which the rays impinge upon the mirror movesaway from the centre of the field. The rate of increase depends upon thef number of the mirror.

[0015] A practical arrangement in which the problem of aberration can bereduced is illustrated in FIG. 2 of the drawings and is formed from twoellipsoidal mirrors (20 and 21). These mirrors are coupled together andso mounted that the output focus of the first mirror (20) coincides withthe input focus of the second mirror (21), this point of coincidencebeing shown at (22). Furthermore, the second mirror (21) is so arrangedthat it reflects in the opposite sense to the first mirror (20). In suchan arrangement, the magnification can be varied by adjusting therelative orientation of the mirrors in such a way that the entry angleof the light rays changes. A positive increase in ray entry angle to thefirst ellipsoidal mirror results in a lower magnification and thisdistorts the wavefront at the intermediate focus (22). This effecthowever is countered at the second ellipsoidal mirror as the change inentry angle tends to increase magnification. Thus, the wavefront isrestored to a certain extent by the second mirror. The wavefrontrestoration is almost perfect when one ellipsoidal mirror has amagnification which is the reciprocal of the other. Thus, aberration isminimal when the system delivers unity magnification. However,aberration will grow relatively slowly as the system magnification isincreased.

[0016]FIG. 2 shows the two ellipsoidal mirrors in a configuration whichprovides low magnification. FIG. 2a shows the two mirrors in a differentconfiguration which provides a relatively high magnification.

[0017] It will therefore be seen that an optical system havingcontinuously variable magnification can be constructed by arranging twoellipsoidal mirrors so that their base plates are pinned together insuch a way that the second focus of the first ellipsoid coincides withthe first focus of the second ellipsoid. The magnification provided bysuch a mirror arrangement can be varied by changing the relativeorientation of the mirrors so that the ray entry angle of light into thesystem is changed. For a given system magnification there will be anoptimum pair of angles between the axes of the mirrors that achieves aminimal image aberration. However, maintaining a precise control ofangle is not a necessary requirement.

[0018] Whilst a system as described above with reference to FIG. 2 canachieve satisfactory results, it does have a drawback in that therelative position and angle of rays into the system changes with respectto the output of the system. This may not be a problem if the imagesensing device which is to receive the radiation of the secondellipsoidal mirror can be moved easily relative to the object, but thereare some systems in which it is necessary to maintain a fixed spatialrelationship between the object and the image. In such a system thearrangement, shown in FIG. 2, would not be satisfactory.

[0019] The drawback referred can be overcome by providing a mirrorarrangement of the type shown in FIG. 3 of the drawings. FIG. 3 of thedrawings illustrates a continuously variable magnification opticalsystem comprising a first pair of ellipsoidal mirrors (30 and 31) and asecond pair of ellipsoidal mirrors (32, 33). The mirror (30) is mountedupon a pivot (34) which is attached to the chassis of the optical systemand the mirror (32) has a pivot point (35) which is also attached to thesystem chassis. The mirrors (31 and 33) have a common intermediate pivotpoint (36) which is capable of moving with respect to the chassis. InFIGS. 3 and 3a, reference point 37 is the image focus for mirror 30 andthe object focus point 31. Reference point 38 is the object focus pointfor mirror 32 and the image focus point for mirror 33.

[0020] In the arrangement shown in FIG. 3 the two pairs of mirrors arelinked in such a way that the output focus of the first pair is pinnedto the input focus of the second pair at point (36). This providessufficient degrees of freedom to change the angles between the axes ofthe four mirrors whilst maintaining the output of the complete system ata fixed point relative to the system input. Furthermore, it is possibleto arrange the geometry such that light rays from an object enter andexit at the same relative angle.

[0021] The pivot point (34) forms the input to the system and the pivotpoint (35) constitutes the output of the system. The pivot point (36) isarranged such that it can move along a predetermined optimal path inorder to achieve different system magnifications. For any given systemmagnification there will be a position for the point (36) which givesthe best aberration performance. In a practical arrangement this pathwill be a compromise between the optimal and what can be achieved by theparticular mechanical system employed.

What is claimed is:
 1. A variable magnification optical systemcomprising: a pair of ellipsoidal mirrors each having an axis andpositioned so that a point at which radiation reflected from the firstmirror of the pair of mirrors is focused corresponds substantially withan input focus of the second mirror of the pair of mirrors.
 2. Theoptical system as set forth in claim 1, wherein the mirrors arepivotably mounted so that an angle subtended between the axes thereofcan be varied.
 3. The optical system as set forth in claim 1, whereinthe magnification of the first ellipsoidal mirror is substantially thereciprocal of the magnification of the second ellipsoidal mirror.
 4. Theoptical system as set forth in claim 2 further comprising: a first pairof ellipsoidal mirrors (30, 31) each having an axis and including: afirst input mirror (30) having an object focus point (34) and an imagefocus point (37); a first output mirror (31) having an object focuspoint (37) and an image focus point (36); wherein the first pair ofellipsoidal mirrors (30, 31) are positioned so that the image focuspoint (37) of the first input mirror (30) corresponds substantially withthe object focus point (37) of the first output mirror (31); a secondpair of ellipsoidal mirrors (32, 33) each having an axis and including:a second input mirror (33) having an object focus point (36) and animage focus point (38); a second output mirror (32) having an objectfocus (38) point and an image focus point (35); wherein the second pairof ellipsoidal mirrors (32, 33) are positioned so that the image focuspoint (38) of the second input mirror (33) corresponds substantiallywith the object focus point (38) of the second output mirror (32);wherein the image focus point (36) of the first output mirror (31) issubstantially the same as the object focus point of the second inputmirror (33); means for adjusting the relative position between the firstand second pairs of ellipsoidal mirrors (31, 32, 33, 34) wherein theimage focus point (36) of the first output mirror (31) is constrained tomove along a predetermined path to achieve a change in magnification.